San Francisco 49ers superstar Aldon Smith was just arrested at LAX after allegedly yelling "bomb" in the airport terminal ... TMZ Sports has learned.

Witnesses at the airport say Smith was very loud when cops arrived to the scene.

We're told the drama started at the gate, where Smith was trying to board a plane. We're told he got into an argument with one of the people working at the gate.

Smith has a long history with the law -- the 24-year-old defensive end was arrested in 2012 for DUI and in 2013, he was charged with 3 counts of illegal possession of an assault weapon stemming from a violent incident at his home in S